When no oxygen is present, anaerobic respiration occurs.It is the only way to produce ATP in this condition.NAD+ is necessary because it is converted into NADH and at the same time ADP is converted to ATP in the reaction of pyruvate synthesis from glucose.Therefore, NADH must be converted to NAD+ in order to have NAD+ always present in the cell.
emigration would theoretically reduce the genetic variation as unique individuals leave, but immigration would bring unique traits into circulation in an ecosystem.
I'm not super great at biogy tho so this might not be right... :)
